Rotorcraft are fitted with at least one rotor that is driven by an engine, in particular a diesel engine. The rotary connection between the engine and the rotor takes place via a junction between a drive shaft engaged with the engine and a driven shaft engaged with the rotor. The engagement of the drive and/or driven shafts respectively with the engine and with the rotor may be direct engagements or indirect engagements via mechanisms, such as transmission mechanisms in particular. Such intermediate mechanisms may be interposed between the drive and/or driven shaft(s) and the corresponding members with which they are respectively engaged.
A general problem occurs that lies in torque pulsations that are naturally induced between the drive shaft and the driven shaft. Such torque pulsations are particularly large in rotorcraft because of the weight of such aircraft that needs to be compensated in order to lift them and/or move them. More particularly, diesel engines can be used to drive such rotors, and by their architecture they generate very large torque pulsations. The spontaneous combustion of fuel in the combustion chamber gives rise to a sudden increase in pressure that induces instantaneous torque peaks with a spectral response that presents high levels of harmonics up to harmonics of high order. Such torque pulsations transmitted via the transmission system between the engine and the rotor tend to give rise to fatigue failures of the members making up the transmission system.
In the field of transmission between an engine and a member that is to be driven in rotation, various solutions are known for attempting to attenuate such torque pulsations.
A first solution consists in interposing a flywheel of large mass between the drive shaft and the driven shaft in order to attenuate torque pulsations. Such a solution presents the advantage of being simple to implement, but also the drawback of increasing the overall size and weight of the transmission system, and this needs to be avoided in the field of rotorcraft.
A second solution consists in interposing a torsion mechanism between the drive shaft and the driven shaft. Commonly, such a mechanism makes use of the ability of one of the shafts, and more particularly the drive shaft, to accommodate twisting by giving it a certain amount of flexibility. The torsion shaft enables torque pulsations to be absorbed by setting its resonant frequency well below the frequency of the excitation that gives rise to the torque pulsations. Such a solution presents the advantage of being simple to implement, but it requires the use of a shaft presenting a length and/or a weight that makes it unacceptable for use in the field of rotorcraft.